In recent years, volatile materials remaining in polymer compositions have become an issue from the standpoint of hygiene and safety, and polymers that have reduced volatile materials have become demanded as products. Among techniques for removing volatile materials from polymer compositions, particularly, methods for the production of a polymer from a solution of a monomer require a step of removing an unreacted monomer and solvent from the polymer composition, the final product. For instance, such a product having residual styrene and residual solvent in a combined amount of 150 ppm or less in polystyrene, and such a product having a residual nitrile monomer in an amount of 10 ppm or less in a copolymer with a nitrile-series monomer, are desired. In the bulk polymerization of polystyrene, as a means of isolating monomer and volatile materials from the polymer composition, the removal of volatile materials is generally carried out by heating the polymer solution, under reduced pressure, to a temperature higher than the temperature at which the volatile materials vaporize.
However, although the conventional techniques can, for example, reduce the styrene and residual solvent in polystyrene easily to 500 to 1,000 ppm in total, to reduce them to 150 ppm or less, simple heating cannot attain the removal of volatile materials, and various contrivances are needed.
For instance, a method is known in which water is injected into a melted polymer, and then flushing with water is carried out under a reduced pressure, to remove residual volatiles together with the water. For example, U.S. Pat. No. 3,773,740 discloses a technique in which water is added to a polymer melt, so that 0.5 to 2.75% by weight of water is injected, and thereafter the pressure of the melt is reduced to 20 to 40 Torr, to flush with the water all at once, thereby reducing the amount of residual aromatic monomer to 0.3% by weight. This technique, however, cannot carry out a high degree of removal of volatile materials to the level demanded currently. It is conceivable to increase the removal efficiency of volatiles by further reducing the degree of the pressure reduction, but under a vapor pressure of water of less than 4.579 mmHg, water has to be cooled to less than 0.degree. C. in order to condense the water. Accordingly, the water freezes in the condenser under a pressure in the condenser of less than 5 Torr, to lower the condensing ability. Therefore, in order to produce a polymer composition having reduced amounts of residual volatiles continuously, it is required to keep the pressure at 10 Torr or higher.
As a method for solving this problem, for example, a method is disclosed in U.S. Pat. No. 5,380,822, wherein a residual monomer, oligomers (e.g. dimer and trimer), and solvent are reduced to 500 ppm or less, and preferably 150 ppm or less, in at least one polymer, particularly a polymer of a vinyl aromatic monomer, such as a polystyrene, or in a blended composition of a polymer of a vinyl aromatic monomer with a polyphenylene oxide. In this technique, water, in an amount equal to or more than the amount of residual volatiles, i.e. in an amount of 1% by weight or more, is injected into the polymer or polymer composition, whose pressure is kept at 500 to 1,500 psi at 200 to 270.degree. C., which polymer or polymer composition is then introduced into a flash chamber devolatilizer having a pressure-reduced zone, where the pressure is kept at less than 5 Torr, and preferably at less than 3 Torr, thereby allowing the polymer to pass through distributor tray means, to remove volatile materials. In this example, in order to bring residual volatiles to 150 ppm or less, it is required to keep the pressure in the devolatilizing tank at less than 5 Torr, preferably less than 3 Torr, to expose the water and monomer in the polymer composition. Industrially, in order to remove the water and monomer resulting from the devolatilization by condensation, an ejector is placed upstream of the condenser, to keep the pressure in the condenser at 5 Torr or more, so that water may be prevented from freezing.
Further, in U.S. Pat. No. 5,350,813, this problem is solved by adding, instead of water, an organic solvent having a lower freezing point, such as acetone and methanol.